Phosphorescence-sensitized delayed fluorescence light emitting system

ABSTRACT

Disclosed is a device that includes an emissive material or region including a host that is doped with a first material as an emitter that is an acceptor and a phosphorescent-capable second material as a sensitizer. The first material and the second material each has a first singlet state and a first triplet state. The first triplet state of the second material is not lower than the first triplet state of the first material. The second material transfers excitons to the first material and the excitons that transition to the first triplet state of the first material can be activated to the first singlet state of the first material through a thermal activation process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. application Ser.No. 13/794,937, filed Mar. 12, 2013, which claims priority to U.S.Provisional Application Ser. No. 61/697,986, filed Sep. 7, 2012, theentire contents of which are incorporated herein by reference.

JOINT RESEARCH AGREEMENT

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to a joint universitycorporation research agreement: Regents of the University of Michigan,Princeton University, The University of Southern California, and theUniversal Display Corporation. The agreement was in effect on and beforethe date the claimed invention was made, and the claimed invention wasmade as a result of activities undertaken within the scope of theagreement.

FIELD OF THE INVENTION

The present invention relates to light emitting devices and, morespecifically, to organic light emitting devices that may includephosphorescent-sensitized emissive layers.

BACKGROUND

Opto-electronic devices that make use of organic materials are becomingincreasingly desirable for a number of reasons. Many of the materialsused to make such devices are relatively inexpensive, so organicopto-electronic devices have the potential for cost advantages overinorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Examples of organic opto-electronic devices include organic lightemitting devices (OLEDs), organic phototransistors, organic photovoltaiccells, and organic photodetectors. For OLEDs, the organic materials mayhave performance advantages over conventional materials. For example,the wavelength at which an organic emissive layer emits light maygenerally be readily tuned with appropriate dopants.

OLEDs make use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as flat paneldisplays, illumination, and backlighting. Several OLED materials andconfigurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and5,707,745, which are incorporated herein by reference in their entirety.

One application for phosphorescent emissive molecules is a full colordisplay. Industry standards for such a display call for pixels adaptedto emit particular colors, referred to as “saturated” colors. Inparticular, these standards call for saturated red, green, and bluepixels. Color may be measured using CIE coordinates, which are wellknown to the art.

One example of a green emissive molecule is tris(2-phenylpyridine)iridium, denoted Ir(ppy)₃, which has the following structure:

In this, and later figures herein, we depict the dative bond fromnitrogen to metal (here, Ir) as a straight line.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules.

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. Where a first layer isdescribed as “disposed over” a second layer, the first layer is disposedfurther away from substrate. There may be other layers between the firstand second layer, unless it is specified that the first layer is “incontact with” the second layer. For example, a cathode may be describedas “disposed over” an anode, even though there are various organiclayers in between.

As used herein, “solution processable” means capable of being dissolved,dispersed, or transported in and/or deposited from a liquid medium,either in solution or suspension form.

A ligand may be referred to as “photoactive” when it is believed thatthe ligand directly contributes to the photoactive properties of anemissive material. A ligand may be referred to as “ancillary” when it isbelieved that the ligand does not contribute to the photoactiveproperties of an emissive material, although an ancillary ligand mayalter the properties of a photoactive ligand.

As used herein, and as would be generally understood by one skilled inthe art, a first “Highest Occupied Molecular Orbital” (HOMO) or “LowestUnoccupied Molecular Orbital” (LUMO) energy level is “greater than” or“higher than” a second HOMO or LUMO energy level if the first energylevel is closer to the vacuum energy level. Since ionization potentials(IP) are measured as a negative energy relative to a vacuum level, ahigher HOMO energy level corresponds to an IP having a smaller absolutevalue (an IP that is less negative). Similarly, a higher LUMO energylevel corresponds to an electron affinity (EA) having a smaller absolutevalue (an EA that is less negative). On a conventional energy leveldiagram, with the vacuum level at the top, the LUMO energy level of amaterial is higher than the HOMO energy level of the same material. A“higher” HOMO or LUMO energy level appears closer to the top of such adiagram than a “lower” HOMO or LUMO energy level.

As used herein, and as would be generally understood by one skilled inthe art, a first work function is “greater than” or “higher than” asecond work function if the first work function has a higher absolutevalue. Because work functions are generally measured as negative numbersrelative to vacuum level, this means that a “higher” work function ismore negative. On a conventional energy level diagram, with the vacuumlevel at the top, a “higher” work function is illustrated as furtheraway from the vacuum level in the downward direction. Thus, thedefinitions of HOMO and LUMO energy levels follow a different conventionthan work functions.

More details on OLEDs, and the definitions described above, can be foundin U.S. Pat. No. 7,279,704, which is incorporated herein by reference inits entirety.

SUMMARY OF THE INVENTION

In an embodiment, a region of a device is provided that includes a firstmaterial and a second material. The first and second materials may beco-dopants of an emissive material or region. The first material mayhave an energy gap of not more than about 100 meV between the firstexcited singlet state and the first excited triplet state. The secondmaterial may be a phosphorescent-capable material, and may act as asensitizer to the first material. The first triplet state of the secondmaterial may not be lower than the first triplet state of the firstmaterial. The first material may exhibit E-type delayed fluorescence. Itmay include a metal complex, such as a Cu complex, an organic compound,such as a donor-acceptor type material, or combinations thereof. Devicesthat include the region may include full-color displays, mobile devices,and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an organic light emitting device.

FIG. 2 shows an inverted organic light emitting device that does nothave a separate electron transport layer.

FIG. 3 shows an example energy diagram for a sensitizer-acceptor system.

FIG. 4 shows an example energy diagram for a sensitizer-acceptor systemhaving a material with a relatively small energy gap according to anembodiment.

DETAILED DESCRIPTION

Generally, an OLED comprises at least one organic layer disposed betweenand electrically connected to an anode and a cathode. When a current isapplied, the anode injects holes and the cathode injects electrons intothe organic layer(s). The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, an “exciton,” which is a localizedelectron-hole pair having an excited energy state, is formed. Light isemitted when the exciton relaxes via a photoemissive mechanism. In somecases, the exciton may be localized on an excimer or an exciplex.Non-radiative mechanisms, such as thermal relaxation, may also occur,but are generally considered undesirable.

The initial OLEDs used emissive molecules that emitted light from theirsinglet states (“fluorescence”) as disclosed, for example, in U.S. Pat.No. 4,769,292, which is incorporated by reference in its entirety.Fluorescent emission generally occurs in a time frame of less than 10nanoseconds.

More recently, OLEDs having emissive materials that emit light fromtriplet states (“phosphorescence”) have been demonstrated. Baldo et al.,“Highly Efficient Phosphorescent Emission from OrganicElectroluminescent Devices,” Nature, vol. 395, 151-154, 1998;(“Baldo-I”) and Baldo et al., “Very high-efficiency green organiclight-emitting devices based on electrophosphorescence,” Appl. Phys.Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporatedby reference in their entireties. Phosphorescence is described in moredetail in U.S. Pat. No. 7,279,704 at cols. 5-6, which are incorporatedby reference.

FIG. 1 shows an organic light emitting device 100. The figures are notnecessarily drawn to scale. Device 100 may include a substrate 110, ananode 115, a hole injection layer 120, a hole transport layer 125, anelectron blocking layer 130, an emissive layer 135, a hole blockinglayer 140, an electron transport layer 145, an electron injection layer150, a protective layer 155, a cathode 160, and a barrier layer 170.Cathode 160 is a compound cathode having a first conductive layer 162and a second conductive layer 164. Device 100 may be fabricated bydepositing the layers described, in order. The properties and functionsof these various layers, as well as example materials, are described inmore detail in U.S. Pat. No. 7,279,704 at cols. 6-10, which areincorporated by reference.

More examples for each of these layers are available. For example, aflexible and transparent substrate-anode combination is disclosed inU.S. Pat. No. 5,844,363, which is incorporated by reference in itsentirety. An example of a p-doped hole transport layer is m-MTDATA dopedwith F.sub.4-TCNQ at a molar ratio of 50:1, as disclosed in U.S. PatentApplication Publication No. 2003/0230980, which is incorporated byreference in its entirety. Examples of emissive and host materials aredisclosed in U.S. Pat. No. 6,303,238 to Thompson et al., which isincorporated by reference in its entirety. An example of an n-dopedelectron transport layer is BPhen doped with Li at a molar ratio of 1:1,as disclosed in U.S. Patent Application Publication No. 2003/0230980,which is incorporated by reference in its entirety. U.S. Pat. Nos.5,703,436 and 5,707,745, which are incorporated by reference in theirentireties, disclose examples of cathodes including compound cathodeshaving a thin layer of metal such as Mg:Ag with an overlyingtransparent, electrically-conductive, sputter-deposited ITO layer. Thetheory and use of blocking layers is described in more detail in U.S.Pat. No. 6,097,147 and U.S. Patent Application Publication No.2003/0230980, which are incorporated by reference in their entireties.Examples of injection layers are provided in U.S. Patent ApplicationPublication No. 2004/0174116, which is incorporated by reference in itsentirety. A description of protective layers may be found in U.S. PatentApplication Publication No. 2004/0174116, which is incorporated byreference in its entirety.

FIG. 2 shows an inverted OLED 200. The device includes a substrate 210,a cathode 215, an emissive layer 220, a hole transport layer 225, and ananode 230. Device 200 may be fabricated by depositing the layersdescribed, in order. Because the most common OLED configuration has acathode disposed over the anode, and device 200 has cathode 215 disposedunder anode 230, device 200 may be referred to as an “inverted” OLED.Materials similar to those described with respect to device 100 may beused in the corresponding layers of device 200. FIG. 2 provides oneexample of how some layers may be omitted from the structure of device100.

The simple layered structure illustrated in FIGS. 1 and 2 is provided byway of non-limiting example, and it is understood that embodiments ofthe invention may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional OLEDs may be achieved by combining the various layersdescribed in different ways, or layers may be omitted entirely, based ondesign, performance, and cost factors. Other layers not specificallydescribed may also be included. Materials other than those specificallydescribed may be used. Although many of the examples provided hereindescribe various layers as comprising a single material, it isunderstood that combinations of materials, such as a mixture of host anddopant, or more generally a mixture, may be used. Also, the layers mayhave various sublayers. The names given to the various layers herein arenot intended to be strictly limiting. For example, in device 200, holetransport layer 225 transports holes and injects holes into emissivelayer 220, and may be described as a hole transport layer or a holeinjection layer. In one embodiment, an OLED may be described as havingan “organic layer” disposed between a cathode and an anode. This organiclayer may comprise a single layer, or may further comprise multiplelayers of different organic materials as described, for example, withrespect to FIGS. 1 and 2 .

Structures and materials not specifically described may also be used,such as OLEDs comprised of polymeric materials (PLEDs) such as disclosedin U.S. Pat. No. 5,247,190 to Friend et al., which is incorporated byreference in its entirety. By way of further example, OLEDs having asingle organic layer may be used. OLEDs may be stacked, for example asdescribed in U.S. Pat. No. 5,707,745 to Forrest et al, which isincorporated by reference in its entirety. The OLED structure maydeviate from the simple layered structure illustrated in FIGS. 1 and 2 .For example, the substrate may include an angled reflective surface toimprove out-coupling, such as a mesa structure as described in U.S. Pat.No. 6,091,195 to Forrest et al., and/or a pit structure as described inU.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated byreference in their entireties.

Unless otherwise specified, any of the layers of the various embodimentsmay be deposited by any suitable method. For the organic layers,preferred methods include thermal evaporation, ink-jet, such asdescribed in U.S. Pat. Nos. 6,013,982 and 6,087,196, which areincorporated by reference in their entireties, organic vapor phasedeposition (OVPD), such as described in U.S. Pat. No. 6,337,102 toForrest et al., which is incorporated by reference in its entirety, anddeposition by organic vapor jet printing (OVJP), such as described inU.S. patent application Ser. No. 10/233,470, which is incorporated byreference in its entirety. Other suitable deposition methods includespin coating and other solution based processes. Solution basedprocesses are preferably carried out in nitrogen or an inert atmosphere.For the other layers, preferred methods include thermal evaporation.Preferred patterning methods include deposition through a mask, coldwelding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819,which are incorporated by reference in their entireties, and patterningassociated with some of the deposition methods such as ink-jet and OVID.Other methods may also be used. The materials to be deposited may bemodified to make them compatible with a particular deposition method.For example, substituents such as alkyl and aryl groups, branched orunbranched, and preferably containing at least 3 carbons, may be used insmall molecules to enhance their ability to undergo solution processing.Substituents having 20 carbons or more may be used, and 3-20 carbons isa preferred range. Materials with asymmetric structures may have bettersolution processability than those having symmetric structures, becauseasymmetric materials may have a lower tendency to recrystallize.Dendrimer substituents may be used to enhance the ability of smallmolecules to undergo solution processing.

Devices fabricated in accordance with embodiments of the presentinvention may further optionally comprise a barrier layer. One purposeof the barrier layer is to protect the electrodes and organic layersfrom damaging exposure to harmful species in the environment includingmoisture, vapor and/or gases, etc. The barrier layer may be depositedover, under or next to a substrate, an electrode, or over any otherparts of a device including an edge. The barrier layer may comprise asingle layer, or multiple layers. The barrier layer may be formed byvarious known chemical vapor deposition techniques and may includecompositions having a single phase as well as compositions havingmultiple phases. Any suitable material or combination of materials maybe used for the barrier layer. The barrier layer may incorporate aninorganic or an organic compound or both. The preferred barrier layercomprises a mixture of a polymeric material and a non-polymeric materialas described in U.S. Pat. No. 7,968,146, PCT Pat. Application Nos.PCT/US2007/023098 and PCT/US2009/042829, which are herein incorporatedby reference in their entireties. To be considered a “mixture”, theaforesaid polymeric and non-polymeric materials comprising the barrierlayer should be deposited under the same reaction conditions and/or atthe same time. The weight ratio of polymeric to non-polymeric materialmay be in the range of 95:5 to 5:95. The polymeric material and thenon-polymeric material may be created from the same precursor material.In one example, the mixture of a polymeric material and a non-polymericmaterial consists essentially of polymeric silicon and inorganicsilicon.

Devices fabricated in accordance with embodiments of the invention maybe incorporated into a wide variety of consumer products, including flatpanel displays, computer monitors, medical monitors, televisions,billboards, lights for interior or exterior illumination and/orsignaling, heads up displays, fully transparent displays, flexibledisplays, laser printers, telephones, cell phones, personal digitalassistants (PDAs), laptop computers, digital cameras, camcorders,viewfinders, micro-displays, vehicles, a large area wall, theater orstadium screen, or a sign. Various control mechanisms may be used tocontrol devices fabricated in accordance with the present invention,including passive matrix and active matrix. Many of the devices areintended for use in a temperature range comfortable to humans, such as18 degrees C. to 30 degrees C., and more preferably at room temperature(20-25 degrees C.).

The materials and structures described herein may have applications indevices other than OLEDs. For example, other optoelectronic devices suchas organic solar cells and organic photodetectors may employ thematerials and structures. More generally, organic devices, such asorganic transistors, may employ the materials and structures.

The terms halo, halogen, alkyl, cycloalkyl, alkenyl, alkynyl, arylkyl,heterocyclic group, aryl, aromatic group, and heteroaryl are known tothe art, and are defined in U.S. Pat. No. 7,279,704 at cols. 31-32,which are incorporated herein by reference.

It has been found that some emissive layers, materials, and devices mayhave relatively short operational lifetimes, especially where relativelyhigh energies and relatively long transient times are involved. Forexample, blue phosphorescent OLEDs (PHOLEDs) may have relatively shortoperational lifetimes.

Embodiments of the invention as disclosed herein may address theseissues to improve the operational lifetime of a layer or device byproviding physical arrangements that allow for transfer of tripletexcitons in the layer to single excitons, thus reducing the chance ofdegradation without negatively affecting exciton utilization. Forexample, in an embodiment triplet excitons in a PHOLED may betransferred to singlet excitons in a fluorescent material within thedevice. Such arrangements may be accomplished, for example, by using aphosphorescent-sensitized system. In an embodiment, it may be desirableto have a relatively small energy gap, such as 100 meV (milli-electronvolts) or less, between the first singlet state and the first tripletstate within a sensitized fluorescent material.

FIG. 3 shows an example energy level diagram for a sensitizer-acceptorsystem, in which a phosphorescent sensitizer is used to sensitize afluorescent acceptor. In this system, the quantum efficiency of a devicethat includes the doped region may be reduced because non-radiationtriplet sites in the fluorescent material may act as quenching sites inthe acceptor material. The related, non-preferred T₁ to S₀ transition inthe acceptor is shown by the dotted transition in FIG. 3 .

More specifically, in the system excitons may transition from the S₁ toT₁ state in the sensitizer material through intersystem crossing, aswill be readily understood by one of skill in the art. Excitons may thentransfer to the S₁ state in the acceptor via Forster transfer, or the T₁state via Dexter transfer. T₁ excitons in the acceptor typically willundergo a non-radiative transition to the S₀ state. While this maydecrease the phosphorescent transient lifetime, it necessarily alsoresults in photon loss within the region.

In an embodiment of the invention as disclosed herein, arrangements witha relatively small energy gap between the S₁ and T₁ states in theacceptor, such as 100 meV or less, may be used. In some embodiments, anenergy gap of 0100 meV, 80 meV, 60 meV, 50 meV, 40 meV, 35 meV, or lessmay be preferred. In such an arrangement, excitons that transition tothe T₁ state of the acceptor can be activated to the S₁ state due to therelatively small energy gap. This thermal activation process is fastenough that non-radiative decay from the T₁ state to the S₀ state isminimal or negligible, thus allowing for sensitization up to andincluding 100%. The relatively high level of sensitization may allow fordirect emission from the PHOLED to be reduced or eliminated relative toconventional sensitized devices. In addition, PHOLEDs according toembodiments disclosed herein may make use of conventional structures andmaterials regardless of energy level, other than the specific energylevels and structural differences disclosed herein.

FIG. 4 shows an example energy level diagram in a sensitizer-acceptorsystem according to an embodiment of the invention disclosed herein. Asshown, the same transitions between a sensitizer and an acceptormaterial may occur as discussed with respect to FIG. 3 . However, theacceptor material includes a relatively small energy gap AE between theS₁ and T₁ states, preferably not more than about 100 meV, morepreferably not more than 80 meV, more preferably not more than 60 meV,more preferably not more than 35-40 meV. Non-radiative transition fromthe T₁ state to the S₀ state is reduced; instead, excitons are morelikely to transition to the S₁ state in the acceptor, allowing forincreased sensitization and improved operation relative to the systemillustrated in FIG. 3 , without significant PHOLED degradation.

In an embodiment, Cu(I) complexes may be used as an acceptor in asensitizer-acceptor system. Such complexes may exhibit relativelypronounced thermally-activated delay fluorescence of the lowest excitedsinglet state at temperatures within or comparable to ambient. That is,such materials typically may have a relatively small energy gap betweenS₁ and T₁ states, preferably 100 meV or less as previously disclosed.

Specific materials suitable for use as acceptors in embodimentsdisclosed herein include the following non-limiting examples:

Specific materials suitable for use as sensitizer materials inembodiments disclosed herein include the following non-limitingexamples:

An organic light emitting device is also provided. The device mayinclude an anode, a cathode, and an organic emissive layer disposedbetween the anode and the cathode. The organic emissive layer mayinclude a host and a phosphorescent dopant. The device may also includea layer or region that includes a sensitizer-acceptor system asdisclosed herein. More specifically, the device may include a regionhaving a sensitizer material and an acceptor material, where theacceptor material has an energy gap of not more than about 100 meVbetween the S₁ and T₁ states as previously described. The device mayinclude one or more such regions or layers. The particular emissivematerials included in the region otherwise may be selected such that thedevice emits light at a desired wavelength, as will be understood by oneof skill in the art. Devices that incorporate the layers and regionsdisclosed herein may include full-color displays, portable devices, andother emissive devices. The sensitizer material and the acceptormaterial may be provided as dopants in one or more host materials. Eachof the sensitizer and acceptor material may be provided as a dopant in aseparate host material, or each may be provided as a dopant of the samehost material and/or within the same region. Alternatively, thesensitizer material may be doped with the acceptor material.

As used herein, a material may be described as a “phosphorescent” or“phosphorescent capable” material to indicate that the materialgenerally is capable of emitting from the triplet state. In a givendevice, a phosphorescent capable material may emit from the tripletstate, or may transfer energy from the triplet state to anothermaterial, state, layer, energy level, or the like, depending upon theparticular materials and physical configurations used, as will beunderstood by one of skill in the art.

In an embodiment, an emissive layer may include two materials, such aswhere the emissive layer is doped with both materials. The firstmaterial, typically a fluorescent acceptor material, may have arelatively small energy gap between the first singlet state and thefirst triplet state. The energy gap may be not more than about 100 meV,more preferably not more than about 80 meV, more preferably not morethan about 60 meV, more preferably not more than about 35-40 meV. Thesecond material, typically a sensitizer, may be a phosphorescent-capablematerial that transfers energy to the first material. Energy may betransferred, for example, via exciton energy transfer from an excitedstate in the second material to an excited state in the first material.Specific examples of energy transfer include Forster and Dextertransfers. In some cases, the emission spectrum of the second materialmay overlap or partially overlap an absorption spectrum of the firstmaterial.

In an embodiment, the first material may be a fluorescent material, suchas a material that exhibits E-type delayed fluorescence. Specificexamples of suitable fluorescent materials include metal complexes,specifically Cu complexes, and organic compounds such as donor-acceptortype materials. In an embodiment, the second material may include atransition metal complex having at least one ligand or part of theligand if the ligand is more than bidentate, and may include one or moreof the following:

where R_(a), R_(b), R_(c), and R_(d) may represent mono, di, tri, ortetra substitution, or no substitution, and R_(a), R_(b), R_(c), andR_(d) are independently selected from the following: hydrogen,deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy,aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl,aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile,isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinationsthereof, and where two adjacent substituents of R_(a), R_(b), R_(c), andR_(d) may be joined to form a fused ring or form a multidentate ligand.

In general, the layers and regions disclosed herein may be fabricated bydepositing one layer on another over a substrate, as previouslydescribed, such as in reference to FIGS. 1 and 2 . For example, acathode, an emissive layer including a sensitizer-acceptor system asdescribed, and an anode may be deposited in a stack over a substrate.The layers may be deposited using any suitable technique, as previouslydescribed.

Combination with Other Materials

The materials described herein as useful for a particular layer in anorganic light emitting device may be used in combination with a widevariety of other materials present in the device. For example, emissivedopants disclosed herein may be used in conjunction with a wide varietyof hosts, transport layers, blocking layers, injection layers,electrodes and other layers that may be present. The materials describedor referred to below are non-limiting examples of materials that may beuseful in combination with the compounds disclosed herein, and one ofskill in the art can readily consult the literature to identify othermaterials that may be useful in combination.

HIL/HTL:

A hole injecting/transporting material to be used in the presentinvention is not particularly limited, and any compound may be used aslong as the compound is typically used as a hole injecting/transportingmaterial. Examples of the material include, but not limit to: aphthalocyanine or porphryin derivative; an aromatic amine derivative; anindolocarbazole derivative; a polymer containing fluorohydrocarbon; apolymer with conductivity dopants; a conducting polymer, such asPEDOT/PSS; a self-assembly monomer derived from compounds such asphosphonic acid and silane derivatives; a metal oxide derivative, suchas MoO_(x); a p-type semiconducting organic compound, such as1,4,5,8,9,12-Hexaazatriphenylenehexacarbonitrile; a metal complex, and across-linkable compounds.

Examples of aromatic amine derivatives used in HIL or HTL include, butnot limit to the following general structures:

Each of Ar¹ to Ar⁹ is selected from the group consisting aromatichydrocarbon cyclic compounds such as benzene, biphenyl, triphenyl,triphenylene, naphthalene, anthracene, phenalene, phenanthrene,fluorene, pyrene, chrysene, perylene, azulene; group consisting aromaticheterocyclic compounds such as dibenzothiophene, dibenzofuran,dibenzoselenophene, furan, thiophene, benzofuran, benzothiophene,benzoselenophene, carbazole, indolocarbazole, pyridylindole,pyrrolodipyridine, pyrazole, imidazole, triazole, oxazole, thiazole,oxadiazole, oxatriazole, dioxazole, thiadiazole, pyridine, pyridazine,pyrimidine, pyrazine, triazine, oxazine, oxathiazine, oxadiazine,indole, benzimidazole, indazole, indoxazine, benzoxazole, benzisoxazole,benzothiazole, quinoline, isoquinoline, cinnoline, quinazoline,quinoxaline, naphthyridine, phthalazine, pteridine, xanthene, acridine,phenazine, phenothiazine, phenoxazine, benzofuropyridine,furodipyridine, benzothienopyridine, thienodipyridine,benzoselenophenopyridine, and selenophenodipyridine; and groupconsisting 2 to 10 cyclic structural units which are groups of the sametype or different types selected from the aromatic hydrocarbon cyclicgroup and the aromatic heterocyclic group and are bonded to each otherdirectly or via at least one of oxygen atom, nitrogen atom, sulfur atom,silicon atom, phosphorus atom, boron atom, chain structural unit and thealiphatic cyclic group. Wherein each Ar is further substituted by asubstituent selected from the group consisting of hydrogen, deuterium,halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy,amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl,heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile,isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinationsthereof.

In one aspect, Ar¹ to Ar⁹ is independently selected from the groupconsisting of:

k is an integer from 1 to 20; X¹ to X⁸ is C (including CH) or N; Ar¹ hasthe same group defined above.

Examples of metal complexes used in HIL or HTL include, but not limit tothe following general formula:

M is a metal, having an atomic weight greater than 40; (Y¹-Y²) is abidentate ligand, Y¹ and Y² are independently selected from C, N, O, P,and S; L is an ancillary ligand; m is an integer value from 1 to themaximum number of ligands that may be attached to the metal; and m+n isthe maximum number of ligands that may be attached to the metal.

In one aspect, (Y¹-Y²) is a 2-phenylpyridine derivative.

In another aspect, (Y¹-Y²) is a carbene ligand.

In another aspect, M is selected from Ir, Pt, Os, and Zn.

In a further aspect, the metal complex has a smallest oxidationpotential in solution vs. Fc⁺/Fc couple less than about 0.6 V.

Host:

The light emitting layer of the organic EL device of the presentinvention preferably contains at least a metal complex as light emittingmaterial, and may contain a host material using the metal complex as adopant material. Examples of the host material are not particularlylimited, and any metal complexes or organic compounds may be used aslong as the triplet energy of the host is larger than that of thedopant. While the Table below categorizes host materials as preferredfor devices that emit various colors, any host material may be used withany dopant so long as the triplet criteria is satisfied.

Examples of metal complexes used as host are preferred to have thefollowing general formula:

M is a metal; (Y³-Y⁴) is a bidentate ligand, Y³ and Y⁴ are independentlyselected from C, N, O, P, and S; L is an ancillary ligand; m is aninteger value from 1 to the maximum number of ligands that may beattached to the metal; and m+n is the maximum number of ligands that maybe attached to the metal.

In one aspect, the metal complexes are:

(O—N) is a bidentate ligand, having metal coordinated to atoms O and N.

In another aspect, M is selected from Ir and Pt.

In a further aspect, (Y³-Y⁴) is a carbene ligand.

Examples of organic compounds used as host are selected from the groupconsisting aromatic hydrocarbon cyclic compounds such as benzene,biphenyl, triphenyl, triphenylene, naphthalene, anthracene, phenalene,phenanthrene, fluorene, pyrene, chrysene, perylene, azulene; groupconsisting aromatic heterocyclic compounds such as dibenzothiophene,dibenzofuran, dibenzoselenophene, furan, thiophene, benzofuran,benzothiophene, benzoselenophene, carbazole, indolocarbazole,pyridylindole, pyrrolodipyridine, pyrazole, imidazole, triazole,oxazole, thiazole, oxadiazole, oxatriazole, dioxazole, thiadiazole,pyridine, pyridazine, pyrimidine, pyrazine, triazine, oxazine,oxathiazine, oxadiazine, indole, benzimidazole, indazole, indoxazine,benzoxazole, benzisoxazole, benzothiazole, quinoline, isoquinoline,cinnoline, quinazoline, quinoxaline, naphthyridine, phthalazine,pteridine, xanthene, acridine, phenazine, phenothiazine, phenoxazine,benzofuropyridine, furodipyridine, benzothienopyridine,thienodipyridine, benzoselenophenopyridine, and selenophenodipyridine;and group consisting 2 to 10 cyclic structural units which are groups ofthe same type or different types selected from the aromatic hydrocarboncyclic group and the aromatic heterocyclic group and are bonded to eachother directly or via at least one of oxygen atom, nitrogen atom, sulfuratom, silicon atom, phosphorus atom, boron atom, chain structural unitand the aliphatic cyclic group. Wherein each group is furthersubstituted by a substituent selected from the group consisting ofhydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl,alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl,alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester,nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, andcombinations thereof.

In one aspect, host compound contains at least one of the followinggroups in the molecule:

R¹ to R⁷ is independently selected from the group consisting ofhydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl,alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl,alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester,nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, andcombinations thereof, when it is aryl or heteroaryl, it has the similardefinition as Ar's mentioned above.

k is an integer from 0 to 20.

X¹ to X⁸ is selected from C (including CH) or N.

Z¹ and Z² is selected from NR¹, O, or S.

HBL:

A hole blocking layer (HBL) may be used to reduce the number of holesand/or excitons that leave the emissive layer. The presence of such ablocking layer in a device may result in substantially higherefficiencies as compared to a similar device lacking a blocking layer.Also, a blocking layer may be used to confine emission to a desiredregion of an OLED.

In one aspect, compound used in HBL contains the same molecule or thesame functional groups used as host described above.

In another aspect, compound used in HBL contains at least one of thefollowing groups in the molecule:

k is an integer from 0 to 20; L is an ancillary ligand, m is an integerfrom 1 to 3.

ETL:

Electron transport layer (ETL) may include a material capable oftransporting electrons. Electron transport layer may be intrinsic(undoped), or doped. Doping may be used to enhance conductivity.Examples of the ETL material are not particularly limited, and any metalcomplexes or organic compounds may be used as long as they are typicallyused to transport electrons.

In one aspect, compound used in ETL contains at least one of thefollowing groups in the molecule:

R¹ is selected from the group consisting of hydrogen, deuterium, halide,alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino,silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl,acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl,sulfinyl, sulfonyl, phosphino, and combinations thereof, when it is arylor heteroaryl, it has the similar definition as Ar's mentioned above.

Ar¹ to Ar³ has the similar definition as Ar's mentioned above.

k is an integer from 0 to 20.

X¹ to X⁸ is selected from C (including CH) or N.

In another aspect, the metal complexes used in ETL contains, but notlimit to the following general formula:

(O—N) or (N—N) is a bidentate ligand, having metal coordinated to atomsO, N or N, N; L is an ancillary ligand; m is an integer value from 1 tothe maximum number of ligands that may be attached to the metal.

In any above-mentioned compounds used in each layer of the OLED device,the hydrogen atoms can be partially or fully deuterated. Thus, anyspecifically listed substituent, such as, without limitation, methyl,phenyl, pyridyl, etc. encompasses undeuterated, partially deuterated,and fully deuterated versions thereof. Similarly, classes ofsubstituents such as, without limitation, alkyl, aryl, cycloalkyl,heteroaryl, etc. also encompass undeuterated, partially deuterated, andfully deuterated versions thereof.

In addition to and/or in combination with the materials disclosedherein, many hole injection materials, hole transporting materials, hostmaterials, dopant materials, exiton/hole blocking layer materials,electron transporting and electron injecting materials may be used in anOLED. Non-limiting examples of the materials that may be used in an OLEDin combination with materials disclosed herein are listed in Table XXXbelow. Table XXX lists non-limiting classes of materials, non-limitingexamples of compounds for each class, and references that disclose thematerials.

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It is understood that the various embodiments described herein are byway of example only, and are not intended to limit the scope of theinvention. For example, many of the materials and structures describedherein may be substituted with other materials and structures withoutdeviating from the spirit of the invention. The present invention asclaimed may therefore include variations from the particular examplesand preferred embodiments described herein, as will be apparent to oneof skill in the art. It is understood that various theories as to whythe invention works are not intended to be limiting.

We claim:
 1. A device comprising: an emissive layer, said emissive layercomprising: a host material doped with a first material as an emitterthat is an acceptor and a phosphorescent-capable second material as asensitizer; wherein the first material and the second material each hasa first singlet state and a first triplet state; wherein the firsttriplet state of the second material is not lower than the first tripletstate of the first material; wherein, in operational state at roomtemperature, the second material transfers excitons to the first singletstate and the triplet state of the first material; and wherein theexcitons that are transferred to the first triplet state of the firstmaterial are activated to the first singlet state of the first materialthrough a thermal activation process.
 2. The device of claim 1, whereinthe first material has an energy gap of not more than 100 meV betweenthe first singlet state and the first triplet state in the firstmaterial.
 3. The device of claim 1, wherein the first material comprisesa material that exhibits E-type delayed fluorescence.
 4. The device ofclaim 1, wherein the second material comprises a material having anemission spectrum that at least partially overlaps with an absorptionspectrum of the first material.
 5. The device of claim 1, wherein thefirst material comprises a donor-acceptor type material.
 6. The deviceof claim 1, wherein each of the first material and the second materialis a dopant in a region of the emissive layer.
 7. The device of claim 1,wherein the second material is a transition metal complex having atleast one ligand or part of the ligand if the ligand is more thanbidentate selected from the group consisting of:

wherein R_(a), R_(b), R_(c), and R_(d) may represent mono, di, tri, ortetra substitution, or no substitution; wherein R_(a), R_(b), R_(c), andR_(d) are independently selected from the group consisting of hydrogen,deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy,aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl,aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile,isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinationsthereof; and wherein two adjacent substituents of R_(a), R_(b), R_(c),and R_(d) are optionally joined to form a fused ring or form amultidentate ligand.
 8. An emissive layer in a device, said emissivelayer comprising: a host material doped with a first material as anemitter that is an acceptor and a phosphorescent-capable second materialas a sensitizer; wherein the first material and the second material eachhas a first singlet state and a first triplet state; wherein the firsttriplet state of the second material is not lower than the first tripletstate of the first material; wherein, in operational state at roomtemperature, the second material transfers excitons to the first singletstate and the first triplet state of the first material; and wherein theexcitons that are transferred to the first triplet state of the firstmaterial are activated to the first singlet state of the first materialthrough a thermal activation process.
 9. The emissive layer of claim 8,wherein the first material has an energy gap of not more than 100 meVbetween the first singlet state and the first triplet state.
 10. Theemissive layer of claim 8, wherein the first material has an energy gapof not more than 80 meV between the first singlet state and the firsttriplet state.
 11. The emissive layer of claim 8, wherein the firstmaterial comprises a material that exhibits E-type delayed fluorescence.12. The emissive layer of claim 8, wherein the second material comprisesa material having an emission spectrum that at least partially overlapswith an absorption spectrum of the first material.
 13. The emissivelayer of claim 8, wherein the first material comprises a donor-acceptortype material.
 14. The emissive layer of claim 8, wherein each of thefirst material and the second material is a dopant in a region of theemissive layer.
 15. The emissive layer of claim 8, wherein the secondmaterial is a transition metal complex having at least one ligand orpart of the ligand if the ligand is more than bidentate selected fromthe group consisting of:

wherein R_(a), R_(b), R_(c), and R_(d) may represent mono, di, tri, ortetra substitution, or no substitution; wherein R_(a), R_(b), R_(c), andR_(d) are independently selected from the group consisting of hydrogen,deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy,aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl,aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile,isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinationsthereof; and wherein two adjacent substituents of R_(a), R_(b), R_(c),and R_(d) are optionally joined to form a fused ring or form amultidentate ligand.
 16. A consumer product comprising a device thatcomprises an emissive layer, said emissive layer comprising: a hostmaterial doped with a first material as an emitter that is an acceptorand a phosphorescent-capable second material as a sensitizer; whereinthe first material and the second material each has a first singletstate and a first triplet state; wherein the first triplet state of thesecond material is not lower than the first triplet state of the firstmaterial; wherein, in operational state at room temperature, the secondmaterial transfers excitons to the first singlet state and the firsttriplet state of the first material; and wherein the excitons that aretransferred to the first triplet state of the first material areactivated to the first singlet state of the first material through athermal activation process.
 17. The consumer product of claim 16,wherein the first material comprises a material that exhibits E-typedelayed fluorescence.
 18. The consumer product of claim 16, wherein thesecond material comprises a material having an emission spectrum that atleast partially overlaps with an absorption spectrum of the firstmaterial.
 19. The consumer product of claim 16, wherein the firstmaterial comprises a donor-acceptor type material.
 20. The consumerproduct of claim 16, wherein the consumer product is one of flat paneldisplays, computer monitors, medical monitors, televisions, billboards,lights for interior or exterior illumination and/or signaling, heads updisplays, fully transparent displays, flexible displays, laser printers,telephones, cell phones, personal digital assistants (PDAs), laptopcomputers, digital cameras, camcorders, viewfinders, micro-displays,vehicles, wall screens, theater screens, stadium screens, and signs.